Safety valves utilized in fluid systems where the temperature of the fluid is not significantly high commonly employ a valve sealing surface or ring formed of a soft material such as rubber or plastic because such material provides a pliable mating surface which forms a tight seal, a valve of this type being disclosed for example in U.S. Pat. No. 3,623,699, issued Nov. 30, 1971, to Matousek. However, where safety valves are placed in a fluid system in which the temperature of the fluid is substantially raised, the use of such soft materials at the mating surfaces of these valves has been virtually non-existent because such materials have an extremely high coefficient of expansion which cause it to "flow" and become distorted when exposed to the high temperature fluid in the system, whereby the mating surfaces of the valve do not seat properly. As a result, substantially all high temperature safety valves currently employ metal-to-metal mating surfaces, but valves having metal-to-metal mating surfaces present additional problems in that they are expensive to produce because of the extremely close tolerances which require lapping to obtain the necessary flatness to form an adequate seal at the mating surfaces, and in that even with precision lapping there is a likelihood of some leakage at the mating surfaces particularly when small foreign particles become entrapped between the smooth seating surfaces of the valve and cause marring thereof as a result of the slight lateral movement of the surfaces with respect to one another during seating and unseating of the valve member.
In an attempt to overcome the above-discussed problems, it has been heretofore proposed in U.S. Pat. No. 2,641,278, issued June 9, 1953, to Eplett et al., to provide a molded valve member which is formed entirely of nylon, and which is unconfined peripherally so as to permit the radial expansion of the valve member caused by the temperature of the fluid and the very high coefficient of expansion of nylon. Because the valve member is made entirely of nylon, the substantial expansion of the valve member would make it prohibitive in many high temperature systems, and the patent itself suggests that the valve would not be effective in systems where the fluid temperature exceeds 300.degree. F.
The present invention provides a safety valve which avoids the aforementioned drawbacks of conventional safety valves, and which operates substantially without leakage, even after use, in fluid systems at elevated temperatures.